Gross Pollutant
Traps, or GPTs, are devises used to prevent large items
polluting waterways.

They are used in
stormwater drains, urban wetlands, airports, beach
fronts, industrial plants, flood plains and other
locations. They generally collect larger items from the
water, such as take away containers, leaves, bottles and
plastic bags. Smaller pollutants, such as dirt,
chemicals, heavy metals and bacteria are not collected
directly by the GPTs; however, some small particles are
caught up in the larger items in the trap and thus
prevented from reaching the waterway.

GPT Types

There are two
main categories of GPTs. They are grouped according to
storing a dry or wet load; collected items are either
stored above (dry) or below (wet) standing water levels.

Traps that store
trapped items in a dry state are generally cheaper to
operate as the collected material can be delivered to
local landfill facilities without issue.

Wet loads traps
are more complicated and thus more expensive to operate.
They require suction equipment for cleaning and the wet
wastes are classified as toxic liquids. Disposal is via
an environmentally controlled waste station under strict
guidelines. There is also the risk of further pollution
occurring if the trap is cleaned infrequently;
biochemical reactions take place between pollutants in
the store area and the by-products can be washed into
the waterway, especially in overflow conditions.

Different traps
can be designed using one or a combination of the
following techniques:

vScreening

vStilling or stopping
the flow of water

vFlow separation

vSedimentation

vFlotation

Traps can be
small, such as a screen over an inlet pit, or very large
when it straddles a channel and can have a footprint up
to 20 metres

Design Factors

The design of a
GPT should be specific to the location it is to be used
in. A GPT that works well at the entrance of an urban
wetland will be less effective in the centre of a
concrete work area, for instance.

Traps are
designed to meet the mid rainfall expectations of the
given area; swales may be placed upstream to help cope
with higher rainfall situations.

Some factors to
be considered in GPT design are:

vSize of particles to be
caught in that location

vPhysical space
available for the trap

vFrequency of storms or
other major water influxes

vAverage flow rates over
a year

vMaintenance
requirements – the ease and safety of access for
maintenance work

vFrequency of
maintenance that is practical in the location (obviously
less often in remote places, more often in high
pollution areas)

vEstimated loading in
the area

vSafety and aesthetics
of the trap being exposed or enclosed

vInstallation and
operating costs

Also to be
specified is the maintenance schedule appropriate for
the trap. The size of the trap and the likelihood of
biochemical reactions will affect this schedule, also.

Alongside roads,
litter baskets (especially SEPTs – Side Entry Pollutant
Traps) and trash racks may be used instead of GPTs.
Likewise, floating booms are often an alternative or an
additional measure in low velocity water. Recent studies
have shown that less than 20% of litter floats along the
water so floating booms are of limited use in screening
out large pollutants.

Tash Hughes is
the owner of
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